In August the Birkenhead River begins to clear and water levels slowly drop. As the nights get cooler, the glaciers stop their summer meltdown, and flow from upper watershed tributaries like Poole Creek lessens. The river rafting season winds down. Anxious sockeye pool up at the river’s mouth waiting for the genetically programmed signal to enter the river for their annual spawning ritual.
Meanwhile, patient human observers can discern the dark ghostly shapes barely visible in the clearing waters of the Birkenhead above the new crossing in the braided reaches of the river. These are the chinook salmon, the largest of the species of the Pacific salmon. Numbering approximately 350, they will spawn in the four week period from Aug. 25 to Sept. 30. They are easily distinguished from the much smaller, more numerous red sockeye which spawn in the latter half of September.
Stocks of chinook salmon, distinguished by run timing and location of spawning grounds, are genetically distinct in different watersheds. In the case of the Fraser River stocks, which includes the Birkenhead, the differences reflect an interesting history of colonization and genetic evolution.
Taking ourselves back about 15,000 years, we find B.C. covered with a thick layer of ice. Fast forward about 5,000 years and we are in a gradual melting period. A huge lake has formed at the present location of Kamloops. It extends from Deadman Creek west of Kamloops, northward up the north Thompson and fills the South Thompson valley. It is known posthumously as Thompson Lake and was formed behind a huge ice jam at the current Hells Gate site. The topography at the time dictated that the waters from the Upper Fraser flowed through this lake into the Columbia Basin via the Okanagan and Shuswap watersheds.
Columbia River chinook, having survived the ice age in the Columbia estuary, now migrate slowly up the Columbia one generation at a time. Eventually, they colonize the whole upper Fraser watershed into genetically distinct stocks in the various Fraser tributaries. All salmon species return to the their natal streams to spawn but a very small percentage of individuals stray to colonize new streams. Genetically controlled local adaptations such as maturity, run timing into fresh water and at the spawning grounds can evolve quickly, within less than 50 generations (250 years in the case of chinook).
Meanwhile, in the Lower Fraser more colonization is going on. A select few tributaries become colonized directly from the Fraser estuary. One of these is the Birkenhead River. The result of this history of colonization and genetic interaction between stocks is a unique Birkenhead River stock of chinook. They are white fleshed, have the earliest run timing of all the Fraser River stocks (they enter the Fraser River in March and the Birkenhead in May, hence the term "spring salmon"). They have the most northerly ocean migration, into Alaskan waters. They are "stream" types, which means that the fry spend a year in fresh water before the annual smolt migration to the ocean. They also have the lowest gene flow from other chinook populations of all the other Fraser chinook stocks.
Should the run of Birkenhead River chinook be lost (loss of habitat and overfishing are the biggest threats), the fish’s uniqueness could be their final demise. Transplanting from other Fraser tributaries in all likelihood would not work. The current program at the Birkenhead River Hatchery in Mount Currie is one method of ensuring that some generations of chinook do survive. It is hoped that the hatchery will help to reduce the threats posed by overfishing and habitat loss, but it is up to people to act as stewards of our resource. By complying with current fishing regulations, helping to protect streamside habitat, and staying out of the river at crucial spawning periods, we can all help to ensure that our unique chinook stock survive indefinitely.
The Pemberton library has a wealth of information on the chinook salmon, including some of the latest studies on salmon populations of the Fraser River watershed.
Monthly Bird Count- Saturday, Sept. 6, 7 a.m. Meet at the end of Lorimer Road at the entrance to the Catholic Church. Join local experts as they keep a running count on local bird species. Everyone welcome.
Written by: Hugh Naylor